Abstract

Nanomaterials have attracted great interest in recent years because of the unusual mechanical, electrical, electronic, optical, magnetic and surface properties. The high surface/volume ratio of these materials has significant implications with respect to energy storage. Both the high surface area and the opportunity for nanomaterial consolidation are key attributes of this new class of materials for hydrogen storage devices. Nanostructured systems including carbon nanotubes, nano-magnesium based hydrides, complex hydride/carbon nanocomposites, boron nitride nanotubes, nanotubes, alanates, polymer nanocomposites, and metal organic frameworks are considered to be potential candidates for storing large quantities of hydrogen. Recent investigations have shown that nanoscale materials may offer advantages if certain physical and chemical effects related to the nanoscale can be used efficiently. The present review focuses the application of nanostructured materials for storing atomic or molecular hydrogen. The synergistic effects of nanocrystalinity and nanocatalyst doping on the metal or complex hydrides for improving the thermodynamics and hydrogen reaction kinetics are discussed. In addition, various carbonaceous nanomaterials and novel sorbent systems (e.g. carbon nanotubes, fullerenes, nanofibers, polyaniline nanospheres and metal organic frameworks etc.) and their hydrogen storage characteristics are outlined.

1. Introduction

The increase in threats from global warming due to the consumption of
fossil fuels requires our planet to adopt new strategies to harness the
inexhaustible sources of energy [1, 2]. Hydrogen is an energy carrier which
holds tremendous promise as a new renewable and clean energy option [3].
Hydrogen is a convenient, safe, versatile fuel source that can be easily
converted to a desired form of energy without releasing harmful emissions.
Hydrogen is the ideal fuel for the future since it significantly reduces the
greenhouse gas emissions, reduces the global dependence on fossil fuels, and
increases the efficiency of the energy conversion process for both internal
combustion engines and proton exchange membrane fuel cells [4, 5]. Hydrogen used
in the fuel cell directly converts the chemical energy of hydrogen into water,
electricity, and heat [6] as represented by

Hydrogen storage cuts across both hydrogen production and hydrogen
applications and thus assumes a critical role in initiating a hydrogen economy [7–10]. For catering
today’s fuel cell cars, the onboard hydrogen storage is inevitable and an
integral part of the system to be reengineered [11, 12]. The critical
properties of the hydrogen storage materials to be evaluated for automotive
applications are (i) light weight, (ii) cost and availability, (iii) high
volumetric and gravimetric density of hydrogen, (iv) fast kinetics, (v) ease of
activation, (vi) low temperature of dissociation or decomposition, (vii)
appropriate thermodynamic properties, (viii) long-term cycling stability, and
(ix) high degree of reversibility. All the said properties greatly demand from
us to understand the fundamental mechanistic behavior of materials involving catalysts
and their physicochemical reaction toward hydrogen at an atomic or molecular
scale.

Various hydrogen storage systems (see Figure 1), such as metal hydrides,
complex hydrides, chemical hydrides, adsorbents and nanomaterials (nanotubes,
nanofibers, nanohorns, nanospheres, and nanoparticles), clathrate hydrates,
polymer nanocomposites, metal organic frameworks, and so on [13–19], have been
explored for onboard hydrogen storage applications. However, none of these
materials qualifies and fulfill all hydrogen storage criteria such as (1) high
hydrogen content (>6.0 wt.%), (2) favorable or tuning thermodynamics (30–55 kJ/mol H2)
(3) operate below 100°C for H2 delivery, (4) onboard refueling option for a hydrogen-based infrastructure, (5) cyclic reversibility (~1000
cycles) at moderate temperatures, and so on. Among the various hydrogen storage
systems, the concept of nanomaterials and their wide applications for energy
storage [20] are discussed in the present paper.

2. Nanostructured Materials

Nanostructured materials have
potential promise in hydrogen storage because of their unique features such as
adsorption on the surface, inter- and intragrain boundaries, and bulk
absorption [21, 22]. Nanostructured and nanoscale materials strongly influence
the thermodynamics and kinetics of hydrogen absorption and dissociation by
increasing the diffusion rate as well as by decreasing the required diffusion
length. Additionally, the materials at the nanoscale offer the possibility of
controlling material tailoring parameters independently of their bulk
counterparts. They also lead to the design of light weight hydrogen storage
systems with better hydrogen storage characteristics.

2.1. Nanocatalyst Doping in Complex Borohydrides

Advanced complex hydrides that are light weight, low cost, and have high-hydrogen
density are essential for onboard vehicular storage [8, 23, 24]. Some of the
complex hydrides with reversible capacities achieved are Alanates [25, 26],
Amides [27, 28], Borohydrides [29–31], and
combinations thereof [32, 33]. The challenging tasks to design and develop the
complex hydrides mandate an optimization and overcoming of kinetic and
thermodynamic limitations [18, 34]. The enhancement of reaction kinetics at low
temperatures and the requirement for high hydrogen storage capacity (>6.0 wt%) of hydrogen storage materials could be made possible by catalytic doping.
If nanostructured materials with high surface area are used as the catalytic
dopants, they may offer several advantages for the physicochemical reactions,
such as surface interactions, adsorption in addition to bulk absorption, rapid
kinetics, low-temperature sorption, hydrogen atom dissociation, and molecular
diffusion via the surface catalyst. The intrinsically large surface areas and
unique adsorbing properties of nanophase catalysts can assist the dissociation
of gaseous hydrogen and the small volume of individual nanoparticles can
produce short diffusion paths to the materials’ interiors. The use of nanosized
dopants enables a higher dispersion of the catalytically active species [35]
and thus facilitates higher mass transfer reactions.

The Zn(BH4)2, as obtained from the mechanochemical
reaction of (LiBH4 + 1/2 ZnCl2), exhibits an endothermic
melting transition at around 80–90°C (DSC signals
are not shown in the figure) and a clear-cut weight loss occurs due to the
thermal hydrogen decomposition at around 120°C. Trial experiments were
conducted by introducing different nanocatalyst (nano-Ni (particle size of 3–10 nm) obtained
from QuantumSphere Inc., Calif, USA)
concentrations ( mol%) in this complex system. It is clearly
discernible from this figure that nanocatalyst doping helps to lower the temperature
of decomposition from 120°C down to 100°C. A
concentration of 3 mol% nano-Ni was found to be optimum for the gravimetric
weight loss due to thermal decomposition at low temperature. It is also confirmed
from the gravimetric analysis that nanocatalyst doping enhances the hydrogen
storage characteristics such as reaction kinetics at low-decomposition
temperatures (). The microstructures of the
nanocatalyst doped complex hydride both in imaging mode and EDS mapping
(distribution of different elements) mode as obtained from SEM are shown in
Figures 3(a) and 3(b).

2.2. Synergistic Behavior of Nanocatalyst Doping and Nanocrystalline Form of

It is generally known that pristine MgH2 theoretically can store
~7.6 wt.% hydrogen [36]. However, so far, magnesium hydride-based materials
have limited practical applications because both hydrogenation and
dehydrogenation reactions are very slow and, hence, relatively high
temperatures are required [23]. Magnesium hydride forms ternary and quaternary
hydride structures by reacting with various transition metals (Fe, Co, Ni, etc.)
and thus improved kinetics. Moreover, the nanoscale version of these transition
metal particles offers an additional hydrogen sorption mechanism via its active
surface sites [36, 37]. In a similar way, the synergistic approach of doping
nanoparticles of Fe and Ti with a few mol% of carbon nanotubes (CNTs) on the
sorption behavior of MgH2 has recently been investigated [38]. The
addition of CNT significantly promotes hydrogen diffusion in the host metal
lattice of MgH2 due to the short pathway length and creation of fast
diffusion channels [39]. The dramatic enhancement of kinetics of MgH2 has also been explored through reaction with small amounts of LiBH4 [40]. Though the MgH2 admixing increases the equilibrium plateau
pressure of LiNH2 [41] or LiBH4 [29], catalytic doping of
these complex hydrides has not yet been investigated.It is generally believed that the role of
the CNT/nanocatalyst on either NaAlH4 or MgH2 is to
stabilize the structure and facilitate a reversible hydrogen storage behavior.

The phenomenon of mechanical milling helps to pulverize the particles of
MgH2 into micro- or nanocrystalline phases and thus leads to
lowering the activation energy of desorption [42]. The height of the activation
energy barrier depends on the surface elements. Without using the catalysts,
the activation energy of absorption corresponds to the activation barrier for
the dissociation of the H2 molecule and the formation of hydrogen
atoms. The activation energies of the H2 sorption for the bulk MgH2,
mechanically milled MgH2 and nanocatalyst-doped MgH2, are
shown in Figures 4(a) and 4(c).

It is undoubtedly seen that the activation barrier has been drastically
lowered by nanocatalyst doping which suggests that the collision frequency
between the H2 molecules and transition metal nanoparticles
increases with decreasing size of the catalyst. In addition, Figure 4 shows the
conceptual model of an MgH2 nanocluster and the distribution of
nanocatalyst over the active surface sites for efficient hydrogen storage.

However, the mechanochemical milling of MgH2 introduces defects
and particle size reduction. Thus, obtained micro-/nanocrystalline MgH2 grains show endothermic hydrogen decomposition (see Figure 6) at an earlier
temperature of 340°C. In addition, to nanoscale formation, the doping by a
nanocatalyst certainly decreases the onset transition temperature by as much as
100°C (Figures 5 and 6).

Carbonaceous
materials are attractive candidates for hydrogen storage because of a
combination of adsorption ability, high specific surface area, pore
microstructure, and low-mass density. In spite of extensive results available
on hydrogen uptake by carbonaceous materials, the actual mechanism of storage
still remains a mystery. The interaction may either be based on van der Walls
attractive forces (physisorption) or on the overlap of the highest occupied
molecular orbital of carbon with occupied electronic wave function of the
hydrogen electron, overcoming the activation energy barrier for hydrogen
dissociation (chemisorption). The physisorption of hydrogen limits the hydrogen-to-carbon
ratio to less than one hydrogen atom per two carbon atoms (i.e., 4.2 mass %).
While in chemisorption, the ratio of two hydrogen atoms per one carbon atom is
realized as in the case of polyethylene [43–45]. Physisorbed
hydrogen has a binding energy normally of the order of 0.1 eV, while
chemisorbed hydrogen has C–H covalent bonding, with a binding energy of more
than 2-3 eV.

Dillon et al. presented the first report on hydrogen storage in carbon nanotubes [46]
and triggered a worldwide tide of research on carbonaceous materials. Hydrogen
can be physically adsorbed on activated carbon and be “packed” on the surface
and inside the carbon structure more densely than if it has just been
compressed. The best results achieved with carbon nanotubes to date confirmed
by the National Renewable Energy Laboratory are hydrogen storage density corresponding to
about 10% of the nanotube weight [47].

In
the present study, carbon nanotubes have been successfully grown by microwave plasma-enhanced
chemical vapor deposition (MPECVD), a well-established method [46, 47]. Figure 7(a) represents the as-grown carbon nanotubes on a substrate using optimized
processing conditions such as temperature, gas flow, gas concentrations, and
pressure. Aligned nanotubes, as seen in Figure 7(b), have been grown to ensure
uniformity in the nanotubes’ dimensions. Various seed materials have been
investigated to grow carbon nanotubes and attempted to determine any effect on the hydrogen sorption capacities.

Fullerenes,
on the other hand, a new form of carbon with close-caged molecular structure
were first reported by Kroto et al. in 1985 [48]. It is a potential hydrogen storage
material based on the ability to react with hydrogen via hydrogenation of
carbon-carbon double bonds. The theory predicts that a maximum of 60 hydrogen
atoms can be attached to
both the inside (endohedrally) and outside (exohedrally) of the fullerene
spherical surface. Thus, a stable C60H60 isomer can be
formed with the theoretical hydrogen content of ~7.7 wt%. It seems that the
fullerene hydride reaction is reversible at high temperatures. The 100%
conversion of C60H60 indicates that 30 moles of H2 gas will be released from each mole of fullerene hydride compound. However,
this reaction is not possible because it requires high temperature, about 823–873 K [49]. Solid
C60 has face-centered cubic lattice at room temperature and its
density is ~1.69 g/sm3. Molecules are freely rotating due to weak
intermolecular interaction. Fullerene is an allotropic modification of carbon.
Fullerene molecules are composed of pentagons and hexagons whose vertexes
contain carbon atoms. Fullerene, C60, is the smallest and the most
stable structure (owing to high degree of its symmetry).

Hydrogen
can be stored in glass microspheres of approximately 50 μm diameter. The microspheres can be filled with
hydrogen by heating them to increase the glass permeability to hydrogen. At
room temperature, a pressure of approximately 25 MPa is achieved resulting in
storage density of 14% mass fraction and 10 kg H2/m3 [49].
At 62 MPa, a bed of glass microspheres can store 20 kg H2/m3.
The release of hydrogen occurs by reheating the spheres to again increase the
permeability.

2.4. Nanocomposite Conducting Polymers

Nanocomposite material
consisting of a polyaniline matrix, which can be functionalized by either
catalytic doping or incorporation of nanovariants, is considered to be a
potential promise for hydrogen storage. It was reported that polyaniline could store as much as 6–8 wt% of hydrogen
[50], which, however, another team of
scientists could not reproduce [51]. Yet another recent study reveals that a
hydrogen uptake of 1.4–1.7 wt% H2 has been reported for the polymers of intrinsic microscopy [52]. Polyaniline is
a conductive polymer, with conductivity on the order of 100 S/cm.
This is higher than that of typical nonconducting polymers, but much lower than
that of metals [53]. In addition to its conductivity, polyaniline emeraldine
base (EB) is very simple and inexpensive to polymerize. It is because of this
simplicity that it was chosen as a matrix material for the nanocomposite
structure.

Figure 8 represents the hydrogen sorption kinetics of polyaniline nanospheres at room
temperature. From this figure, it is discernible that the hydrogen uptake and
release of ~4.0 wt.% occurs in the initial run. However, during the consecutive
cycles, the hydrogen storage capacity and kinetics were decreased. The SEM
microstructure of polyaniline nanospheres are shown in Figure 9(a). Uniform
cluster sizes of 50–100 nm are widely
distributed over the surface. The microstructures after hydrogen sorption
cycles exhibit microchannels or microcrack formation (see Figure 9(b)). This
correlates very well with effective hydrogenation as observed from sorption
kinetic profiles (see Figure 8). Further cyclic reversibility and associated mechanistic
behavior for hydrogen uptake and release kinetics are still underway.

Functionalization (see schematic Figure 10) has been carried out by the introduction of chemical groups into a polymer
molecule or conversion of one chemical group to another group, which leads to a
polymer with chemical, physical, or other functions. Functional polymers
act as a catalyst to bind selectively to particular species, to capture and
transport electric charge or energy, and to convert light into charge carriers
and vice versa.

Figure 10: Schematics for the development of functionalized conducting polymer.

2.5. High-Surface Area Sorbents and New Materials Concepts

There is a pressing need for the
discovery and development of new reversible materials. One new area that may be
promising is that of high-surface area hydrogen sorbents based on microporous
metal-organic frameworks (MOFs). Such materials are synthetic, crystalline, and
microporous and are composed of metal/oxide groups linked together by organic
struts. Hydrogen storage capacity at 78 K (−195°C) has been reported as high as
4 wt% via an adsorptive mechanism, with a room temperature capacity of
approximately 1 wt% [54]. However, due to the highly porous nature of these
materials, volumetric capacity may still be a significant issue.

Another class of materials for hydrogen storage may be
clathrates [15], which are primarily hydrogen-bonded H2O frameworks.
Initial studies have indicated that significant amounts of hydrogen molecules
can be incorporated into the sII clathrate. Such materials may be particularly
viable for off-board storage of hydrogen without the need for high pressure or
liquid hydrogen tanks.

3. Summary

Nanostructured
materials such as nanotubes, nanofibers, and nanospheres show potential promise
for hydrogen storage due to high-surface
area, and they may offer several advantages for the physicochemical reactions,
such as surface interactions, adsorption in addition to bulk absorption, rapid
kinetics, low-temperature sorption, hydrogen atom dissociation, and molecular
diffusion via the surface catalyst. The intrinsically large surface areas and
unique adsorbing properties of nanophase materials can assist the dissociation
of gaseous hydrogen, and the small volume of individual nanoparticles can
produce short diffusion paths to the materials’ interiors. The use of nanosized
dopants enables a higher dispersion of the catalytically active species, and thus facilitates
higher mass transfer reactions. Nanocomposites based on polymer matrix and
functionalized carbon nanotubes possess unique microstructure for physisorption
of hydrogen atom/molecule on the surface and inside the bulk. This review paper
discussed briefly various nanomaterials for hydrogen storage and also presented
hydrogen uptake and release characteristics for polyaniline nanospheres at room
temperature.

Acknowledgments

Financial support from US Department of Energy (Contract no. DE-FG36-04GO14224) and QuantumSphere Inc. is gratefully
acknowledged. The authors also thank Dr. Rakesh Joshi for his comments and suggestions.